Not applicable.
Not applicable.
The field of the invention is pulse width modulated (PWM) controllers and more specifically a method and apparatus for modifying modulating signals as a function of a carrier frequency and/or an electrical operating frequency to minimize sampling related errors in phase shift and magnitude.
One type of commonly designed induction motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source generating currents therein. Often, an adjustable speed drive (ASD) will be positioned between the voltage source and the motor to control motor speed.
Many ASD configurations include a PWM inverter consisting of a plurality of switching devices and a controller for controlling the inverter. Referring to FIG. 1, an exemplary inverter 9 has six switches 12-17. The switches 12-17 are arranged in series pairs between positive and negative DC buses 48 and 49, each pair forming one of three inverter legs 39, 40, and 41. Each switch includes a high speed semiconductor switching device in inverse parallel relationship with a diode. For example, diode 23 is associated with switch 12. Similarly, diodes 25, 27, 24, 26 and 28 are associated with switches 14, 16, 13, 15 and 17, respectively.
A controller 11 is linked to each switch by a separate control line. For example, controller 11 is linked to switch 12 via line 51. Similarly, controller 11 is linked to switches 13,14, 15, 16 and 17 via lines 52,53, 54, 55 and 56, respectively. Controller 11 controls the on and off cycles of the switches 12-17 via lines 51-56.
Referring still to FIG. 1, each leg 39, 40 and 41 is linked to a separate one of three motor terminals 31, 3032, respectively. Referring specifically to leg 39, by triggering switches 12, 13 on and off in a repetitive sequence, terminal 31 and winding 36 linked to leg 39 receives high frequency DC voltage pulses. Similarly, each of legs 40 and 41 are controlled to provide pulses to associated terminals 30 and 32 and hence to windings 35 and 37.
Referring to FIG. 2, an exemplary sequence of high frequency voltage pulses 60 that inverter 9 might provide to terminal 31 can be observed along with an exemplary low frequency alternating fundamental voltage 62 and related alternating current 69. By varying the widths of positive portions 63 of each high frequency pulse relative to the widths of negative portions 64 over a series of high frequency voltage pulses 60, a changing average voltage which alternates sinusoidally is generated. The changing average voltage defines the low frequency alternating voltage 62 that drives motor 19. Low frequency alternating voltage 62 in turn produces low frequency alternating current 69 that lags the voltage by a phase angle "PHgr". By triggering switches 12 and 13 in a regulated sequence, inverter 9 is used to control both the amplitude and frequency of voltage 62 that eventually reach the stator windings (e.g., 36 ).
Referring to FIG. 3a, representative waveforms used to generate trigger signals for leg 39 are illustrated. As well known in the art, a carrier signal or waveform 67 is perfectly periodic and operates at what is known as a carrier frequency fc. A command or modulating voltage waveform 68 is sinusoidal, having a much lower frequency fe and a greater period than carrier signal 67.
Referring also to FIGS. 3b and 3c, an upper trigger signal 72 and a lower trigger signal 74 corresponding to a comparison of waveforms 67 and 68 and for controlling the upper and lower switches 12, 13, respectively, can be observed. The turn-on tu1,tu2 and turn-off to1, to2 trigger times of the upper and lower signals 72, 74 come from the intersections of command waveform 68 and carrier waveform 67.
When command waveform 68 intersects carrier waveform 67 while carrier waveform 67 has a positive slope (i.e. during periods Tp), upper signal 72 goes OFF and lower signal 74 goes ON. When command waveform 68 intersects carrier waveform 67 while carrier waveform 67 has a negative slope (i.e. during periods Tn), upper signal 72 goes ON and lower signal 74 goes OFF. Thus, by comparing carrier waveform 67 to command waveform 68, trigger times are determined.
Early control systems operated using only a single carrier signal frequency which, at the time, addressed most application requirements and was suitable given inverter switching limitations. As switching technology has evolved, however, much higher switching speeds have been realized and hence a much greater range of carrier signal frequencies are now available. With control system evolution it has been recognized that carrier signal frequency can have various advantageous and disadvantageous affects on system control and that, therefore, different carrier frequencies are ideal for different applications. For example, harmonic content in a PWM system has been known to generate audible noise in certain applications. The harmonic content in a PWM system can be altered to some degree by altering the carrier frequency and hence audible noise can typically be tuned out of a system via carrier frequency changes.
As another example, increased carrier frequency sometimes results in reflected voltages that have been known to damage system cabling and/or motor windings (see U.S. Pat. No. 5,831,410 titled xe2x80x9cApparatus used with AC motors for eliminating line voltage reflectionsxe2x80x9d which issued on Feb. 12, 1997 for a detailed explanation of reflected waves). As one other example, when carrier frequencies are increased the number of switching cycles are similarly increased and overall switching losses (e.g., switching losses occur during each switching cycle) and system heating are also increased. As yet another example, as carrier frequency is increased ripple current in the resulting waveforms is reduced appreciably. Thus there are tradeoffs that have to be understood and accounted for when selecting carrier frequency for specific system configurations and applications.
There are many systems today that allow carrier frequency to be altered to address application specific requirements. In addition, there are several applications where carrier frequency is altered on the fly as a function of other operating parameters and intended control requirements. For one example of an application where carrier frequency is altered on the fly, see U.S. patent application Ser. No. 09/956,781 titled xe2x80x9cMethod and Apparatus for Compensating for Device Dynamics by Adjusting Inverter Carrier Frequency xe2x80x9d which was filed on Sep. 20, 2001 and which is commonly owned with the present invention.
Unfortunately, under certain circumstances, on the fly carrier frequency changes have been known to cause system disturbances. To this end, FIG. 4 illustrates an exemplary q-axis torque producing current Iqe and a resulting single phase current Iws where a carrier frequency fc is altered at time xcfx841 from 3 KHz to 4 KHz. As illustrated, when the carrier frequency is altered at time xcfx841, a noticeable current disturbance occurs which shows up in single phase current Iws most noticeably as a magnitude change. Although less noticeable, a phase change also occurs at time xcfx841.
Disturbances like the one illustrated in FIG. 4 occur because of the way in which modulating waveforms are generated for comparison to carrier signals. In this regard, an exemplary modulating waveform generator 200 is illustrated in FIG. 5. The generator 200 receives a command frequency signal xcfx89e in radians/second and two phase synchronous d and q-axis command voltage signals Vqe and Vde (e.g., from a synchronous current frame regulator) and uses those signals to generate three phase modulating waveforms for use by a PWM inverter (see 217 ). To this end, generator 200 includes a sampler 202, an integrator 204, a synchronous to stationary transformer 206, a multiplier 208, a two-to-three phase converter 210 and a carrier frequency selector 211. Although illustrated in FIG. 5, carrier signal generator 213 and PWM inverter 217 are not part of the modulating waveform generator.
As its label implies, carrier frequency selector 211 is used to select the carrier frequency fc either manually or, in the case of more sophisticated systems, automatically, as a function of sensed system operating parameters (e.g., component temperatures, sensed ripple/harmonics, etc.). The carrier frequency fc is provided to each of carrier signal generator 213 and sampler 202. Signal generator 213 uses frequency fc to generate a high frequency carrier signal (e.g., 67 in FIG. 3a) that is provided to inverter 217 for comparison as described above.
In the embodiment described here, it is assumed that sampler 202 is programmed to sample the command frequency xcfx89e once per carrier period. Thus, the sampling frequency fs is equal to the carrier frequency fc and sampler 202 samples frequency xcfx89e every period Ts where period Ts=1/fc. The sampled values are provided to integrator 204 which outputs the integrated value as a phase angle xcex8e. The integrator output is a stepped signal as illustrated at 212 where the output value changes every sampling period Ts.
Stepped phase angle signal xcex8e is provided to synchronous to stationary converter 206 which generates a 2xc3x972 matrix 219 of values in the stationary frame of reference. The two phase synchronous voltages Vqe and Vde form a 2xc3x971 matrix 221 and are multiplied by 2xc3x972 matrix 219 thereby generating two phase stationary frame voltage command values 223 (see also 229). Converter 210 converts the two phase stationary frame voltage command values to three phase modulating signals that are in turn provided to PWM inverter 217 for comparison to the carrier signal.
Referring to FIG. 6, an exemplary ideal fundamental modulating waveform 68 is illustrated along with a sampled or discretized modulating waveform signal 91 that may be produced by multiplier 208 illustrated in FIG. 5. Waveform 68 corresponds to a waveform that would result if the sampling period Ts was a zero or near zero duration.
Referring also to FIG. 7, a small segment of waveform 68 is illustrated along with two separate associated discretized or sampled waveform signals 91 and 93 where the sampled signals correspond to different sampling frequencies. Sampled signal 91 corresponds to a 4 kHz sampling frequency while signal 93 corresponds to an 8 kHz sampling frequency. In addition to waveforms 68, 91 and 93, FIG. 7 also illustrates separate fundamental components of modulating waveforms associated with sampled signals 91 and 93. In FIG. 7, fundamental component 95 corresponds to the 4 kHz sampling signal 91 while fundamental components 97 corresponds to the 8 kHz sampling signal 93.
A simple analysis of FIG. 7 makes clear that the fundamental component of a modulating waveform generated using sampled values of the command frequency xcfx89e is phase shifted by an error angle. In FIG. 7, the phase shift error corresponding to fundamental component 95 is identified as xcex941 while the error corresponding to fundamental component 97 is identified as xcex942. In addition, although not easily observable in either of FIG. 6 or 7, the fundamental components 95 and 97 associated with sampled signals 91 and 93 have lower magnitudes than ideal waveform 68.
While existing commutation algorithms have been developed to compensate for processing delays intrinsic in any electronic components, these algorithms generally do not contemplate compensating for distortion due to sampler related phase and magnitude errors. In many applications where carrier to operating frequency ratios are much greater than unity these commutation algorithms provide suitable results. Unfortunately reflected wave requirements, thermal regulation algorithms and wide speed operation render the great than unity ratio assumption questionable at best.
While reduced modulating waveform magnitudes and phase shifts from ideal waveforms are problematic generally, the affects of these errors on load control are most pronounced when operating frequency or carrier frequency changes occur (see again FIG. 4 where carrier frequency was altered at time xcfx841). Referring again to FIG. 7, as described above, when the carrier frequency fc and hence the sampling frequency fs is 8 kHz, the resulting modulating waveform has a fundamental component 97 and when the carrier and sampling frequency fs are 4 kHz, the resulting modulating waveform has a fundamental component 95. Thus, a carrier frequency change from 8 to 4 kHz or from 4 to 8 kHz results in a fundamental component phase shift identified as xcex943 in FIG. 7. Similar phase shifts occur when other carrier frequency changes occur. While errors occur in the resulting fundamental components as illustrated in FIG. 7, it should be appreciated that distortions occur in other harmonics in the resulting waveforms and should to be corrected.
Thus, there is a need for a controller that can accurately compensate for distortions that occur when carrier frequency used with a PWM controller is altered and to essentially eliminate sampling related phase and magnitude errors.
It has been recognized that the magnitude of the error that results from the sampling/discretizing process is a function of the ratio of the carrier frequency fc to the electrical operating frequency fe. In this regard, experiments have shown that when the ratio of carrier frequency fc to operating frequency fe is large, the magnitude of the phase error resulting therefrom is relatively small and when the ratio is relatively small the magnitude of the phase error resulting therefrom is relatively large. Similarly, when ratio fc/fe is small, the phase distortion that results from a carrier/sampling frequency change is appreciable. Thus, in at least some embodiments of the invention, phase correction is facilitated by tying a correction angle to the carder or sampling frequency or to the ratio of the carrier frequency to the operating frequency.
It has also been recognized that the magnitude error in a modulating waveform generated through sampling is also related to the carrier frequency/operating frequency ratio. Thus, in at least some embodiments of the invention, in addition to a phase correction operation, inventive systems also implement a magnitude correction operation where the magnitude correction is a function of the operating and carrier frequencies.
Consistent with the above, the invention includes a method for use with a controller that samples a command frequency and provides modulating waveforms to a PWM inverter as a function of the sampled command frequency, the inverter also receiving a carrier signal having a carrier frequency, the method for reducing distortions in the modulating waveforms that result from sampling characteristics of the controller, the method comprising the steps of sampling the command frequency at a sampling frequency to generate a series of sampled signals, integrating the sampled signals to generate a phase angle, identifying a correction angle as a function of the sampling frequency, adding the correction angle to the phase angle to generate a corrected phase angle and using the corrected phase angle to generate the modulating waveforms to be provided to the PWM inverter.
In some embodiments the step of identifying a correction angle includes identifying the correction angle as a function of both the sampling frequency and the command frequency. More specifically, the step of identifying a correction angle may include identifying the correction angle as a function of the ratio of the command frequency to the sampling frequency. Even more specifically, the step of identifying a correction angle d may include solving the following equation:
xcex4=xcfx89eTs/2
where xcfx89e is the command frequency in radians/second and Ts is the sampling period.
In some cases the step of using the corrected phase angle to generate the modulating waveforms includes the steps of receiving two phase synchronous voltage command signals, mathematically combining the voltage command signals and the corrected phase angle to generate two phase corrected voltage command signals in the stationary frame of reference and converting the two phase corrected voltage command signals to three phase command signals.
Some embodiments further include the step of identifying a voltage magnitude correction value as a function of the sampling frequency and the step of mathematically combining the voltage command signals and the corrected phase angle may include combining the voltage command signals, the corrected phase angle and the voltage magnitude correction value. Here, the step of combining the voltage command signals, the corrected phase angle and the voltage magnitude correction value may include the steps of multiplying the voltage magnitude correction value by the two phase synchronous voltage command signals to generate corrected two phase voltage command signals, performing a stationary to synchronous conversion on the corrected phase angle to generate a two by two stationary matrix and multiplying the corrected two phase voltage command signals by the two by two stationary matrix to generate the two phase corrected voltage command signals in the stationary frame of reference.
The step of identifying a voltage magnitude correction value as a function of the sampling frequency may include the step of identifying a voltage magnitude correction value as a function of both the sampling frequency and the command frequency. More specifically, the step of identifying a voltage correction magnitude value Vcorr may include the step of solving the following equation:       V    corr    =      1          sin      ⁢              xe2x80x83            ⁢              c        ⁡                  (                                    ω              e                        ⁢                                          T                s                            /              2                                )                    
The invention also includes a method for use with a controller that samples a command frequency and receives two phase synchronous command voltages and provides modulating waveforms to a PWM inverter as a function of the sampled command frequency and command voltages, the inverter also receiving a carrier signal having a carrier frequency, the method for reducing distortions in the modulating waveforms that result from sampling characteristics of the controller, the method comprising the steps of sampling the command frequency at a sampling frequency to generate a series of sampled signals, integrating the sampled signals to operating frequency to the sampling frequency to generate a corrected phase angle, modifying the two phase command voltages as a function of the ratio of the operating frequency to the sampling frequency to generate corrected two phase voltage command signals, mathematically combining the corrected two phase voltage command signals and the corrected phase angle to generate two phase voltage command signals in a stationary frame of reference and converting the two phase voltage command signals in the stationary frame of reference to three phase signals to be provided to the PWM inverter.
Moreover, the invention includes a controller that receives a command frequency and provides modulating waveforms to a PWM inverter as a function of the command frequency, the inverter also receiving a carrier signal having a carrier frequency, the controller for reducing distortions in the modulating waveforms that result from sampling characteristics of the controller, the controller comprising a sampler for sampling the command frequency at a sampling frequency to generate a series of sampled signals, an integrator for integrating the sampled signals to generate a phase angle, a correction angle determiner for identifying a correction angle as a function of the sampling frequency, a summer for adding the correction angle to the phase angle to generate a corrected phase angle and a processor using the corrected phase angle to generate the modulating waveforms to be provided to the PWM inverter.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention.